Xantusiidae is a clade of viviparous (live bearing) lizards that ranges from southwestern North America and Baja California (Xantusia) into Central America (Lepidophyma) and Cuba (Cricosaura). Lepidophyma smithii occurs along the Pacific versant from Guerrero, Mexico to El Salvador. Xantusiidae is a relatively small clade, with 3 genera and approximately 30 living species. Lepidophyma is the most speciose (~17 species), whereas Cricosaura is monotypic. Xantusiids have a reasonably good fossil record extending from the mid-Paleocene onward in western North America.

Xantusiids are fascinating lizards for several reasons. First, although they are almost uniformly diminuitive (Lepidophyma smithii is one of the largest species, with a maximum snout-vent-length of approximately 112 mm), xantusiids generally take several years to reach sexual maturity, and several species give birth to just 1 or 2 offspring. It is a more usual reproductive strategy for small lizards to mature quickly and produce large numbers of offspring, to increase their chances of survival. Despite this low reproductive potential, xantusiid neonates actually have a high life expectancy; this can be attributed at least in part to their secretive lifestyle, which leads to the second reason why xantusiids are particularly interesting -- microhabitat specialization.

Microhabitat specialization is an ecological hallmark of Xantusiidae. Many populations are narrowly restricted to specific niches -- crevices (e.g., Xantusia henshawi in exfoliating granitic cap rocks), interstices in agaves and yuccas in dry climates (Xantusia vigilis), decaying logs in wet climates (e.g., Lepidophyma flavimaculatum) -- and individuals may be found under the same patch of cover throughout their lives! One population of L. smithii in Guerrero inhabits a system of caves and feeds almost solely on the fruit of native figs.

These microhabitat restrictions result in extremely disjunct geographical distributions, and also may be responsible for some morphological convergence within the group (e.g., flattened skulls for crevice dwelling). Xantusiidae also includes two insular endemics: the Cuban Cricosaura typica is the only xantusiid found in the West Indies and is interpreted as one of the Caribbean's few ancient endemic vertebrate lineages; and Xantusia riversiana (formerly Klauberina riversiana) is limited to three of the Channel Islands off the coast of California.

The phylogenetic relationships of Xantusiidae are problematic. Morphology and molecules produce different topologies within the clade: morphology recovers a Cricosaura + Lepidophyma clade, while mitochondrial genes recover a Lepidophyma + Xantusia clade. Lack of resolution of relationships within Xantusiidae has hindered the placement of this clade within the squamate tree. Xantusiidae is a "tree-changing" taxon: it causes homoplasy wherever it is placed, and its placement can tip the balance between the two primary competing hypotheses of scleroglossan relationships. Xantusiidae is traditionally placed within Scincomorpha, but some analyses have placed it near Gekkota. Thus, Xantusiidae is either a highly derived, or extremely basal, scleroglossan clade. Previous analyses of squamate phylogeny have almost certainly suffered in relying on species of the readily available -- but relatively derived -- genus Xantusia as exemplars for Xantusiidae. Cricosaura or a species of Lepidophyma would be more appropriate, but both are exceedingly rare in collections; indeed, some species of Lepidophyma are known from only 1 or 2 specimens.

Whatever the placement of Xantusiidae within squamates, there is no doubt that xantusiids are monophyletic. The following are some of the hypothesized synapomorphies of the lineage (from Estes et al., 1988), most of which can be seen in the skull reconstructions above: supratemporal fenestra closed primarily by postorbital; parietals paired well into postembryonic ontogeny; parietal table extensive posteriorly, largely obscuring braincase in dorsal view, supratemporal process short; vomers fused; ectopterygoid contacts palatine anterolaterally, excluding maxilla from suborbital fenestra; ectopterygoid enlarged medially, restricting suborbital fenestra.

About the Species

This specimen was collected 2.5 miles by road northeast of Acacoyaga, Chiapas, Mexico. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Jessie Maisano of The University of Texas and Dr. Jacques Gauthier of Yale University. Funding for scanning was provided by an NSF grant (DEB-0132227) to Dr. Jack Sites of Brigham Young University. Funding for image processing was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin.

Dorsal view of the scanned specimen.

About this Specimen

The specimen was scanned by Matthew Colbert on 23 September 2002 along the coronal axis for a total of 600 1024x1024 pixel slices. Each slice is 0.0418 mm thick, with an interslice spacing of 0.0418 mm and a field of reconstruction of 18 mm.